The present invention relates to a sensor control system for compensating for degradation of the sensor system. More particularly, the present invention relates to a smart sheet sensor control system which may be provided in an electrophotographic printing machine and in which a reference voltage trigger level is periodically adjusted to always be within a window between a null state indicating an absence of a sheet in a paper location and an activated state indicating a presence of a sheet in the paper location, the adjustment being made so that the reference voltage trigger level is biased toward the null state.
Optical sensors are often used in applications requiring a determination of the presence or absence of a copy sheet or other document passing through a certain paper location by providing a suitable signal in response to a determination of whether or not the sheet is located in the paper location. The optical sensor typically includes a light source whose light beam is directed at the position in which the document is to be sensed. A light sensitive transducer such as a phototransistor or photodiode is mounted in an aligned relationship with the light source to detect the presence or absence of a sheet in the paper location between the light source and the light sensitive transducer.
A common recurring problem in optical sensor systems placed in reproduction machines is degradation of optical sensors in the paper path due to contaminants such as airborne toner particles, paper fibers, carrier particles, dust buildup, and other contaminants, or aging of the light source, a broken optical sensor, or varying thickness, density, color or smoothness of sheets passing through the paper location. Contaminants generally cause failure of the optical system by coating the optical elements and thereby reducing the illumination level at the sensor. This causes a change in an output voltage level used as a signal which identifies whether or not a sheet is present in the paper location between the light source and the light sensitive transducer.
One solution to this problem is to schedule frequent maintenance to clean the sensor and test the level of performance. However, this is extremely expensive due to increased labor costs and increased down time of the machine.
Various solutions have previously been implemented to overcome the above-discussed sensor degradation problem. U.S. Pat. No. 4,097,731 to Krause et al. issued on Jan. 27, 1978 and U.S. Pat. No. 4,097,732 to Krause et al. issued on Jun. 27, 1978 disclose a sensor including apparatus for regulating the intensity of the sensor light source to compensate for extraneous factors in the operating environment such as dust accumulation, component aging and misalignment. Compensating for these factors by adjusting the power output of the lamp is often complex and expensive. Additionally, this type of compensation provides only a limited degree of adjustment.
U.S. Pat. No. 4,670,647 to Fred F. Hubble, III et al. issued on Jun. 7, 1987 is hereby incorporated into this application by reference. U.S. Pat. No. 4,670,647 discloses a compensation method in which the gain of the received signal is automatically adjusted rather than continually adjusting the power out of the light source. This patent includes a document sensor control circuit which compensates for degradation of the sensor by maintaining an output of an amplifier at a constant level. The output of the amplifier is compared with a reference level and the gain of the amplifier is adjusted if the output of the amplifier falls below the reference level.
U.S. Pat. No. 3,789,215 to Penny issued on Jan. 29, 1974 discloses the detection of documents by establishing analog thresholds against which the analog output of a detector is compared. The range of this type of system may be limited and unreliable, since the threshold is adjusted based on a percentage of a previous peak value detected by the photosensitive detection cell. The apparatus of this patent requires a constant measure and continual update of a sample and hold circuit and a compensation for offsets of an amplifier. Additionally, no indication of a required sensor service is made before the sensor has degraded to a point of failure.